Combining
onclick
with
AutoPostBack
can be tricky. By adding an unconditional
return
statement, you end up bypassing the code which causes the page to post back to the server.
Try this:
<asp:CheckBox ID="chkSignIn" runat="server" Text="Sign Out" AutoPostBack="true" OnCheckedChanged="chkSignIn_CheckedChanged" OnClick="if(!confirm('Are you sure you want to mark register?'))return false;" />